1. Technical Field
The present invention relates to an organic electroluminescent device that converts electric energy into light to emit and a production method thereof, and an image display medium. In more detail, the invention relates to an organic electroluminescent device that can be preferably used in the fields of display devices, electronic paper, back lights, illumination light sources, electrophotographic exposure devices, indicators, signboards and the like, a production method thereof and an image display medium.
2. Related Art
Light emission by these electroluminescent devices is a phenomenon in which electrons are injected from one electrode and holes are injected from another electrode, whereby, a light emitting material in an electroluminescent device is excited to a high energy level, and an excited light emitting substance releases, as light, extra energy produced when returned to a ground state. However, in these electroluminescent devices, a driving voltage is still as high as 30 V, densities of electron carriers and hole carriers present in a film are low, and a probability of production of photons by recombination of carriers is low. Therefore, sufficient luminance cannot be obtained, and these electroluminescent devices have not been put to practical use.
However, in 1987, Tang et al reported that in a function separated organic electroluminescent device obtained by sequentially layering a very thin film of a hole transporting organic low molecular weight compound and a very thin film of an electron transporting fluorescent organic low molecular weight compound on a transparent substrate by use of a vacuum vapor deposition method, at such a low voltage as substantially 10 V, such high brightness as 1000 cd/m2 or more can be obtained. Since then, studies and developments of organic electroluminescent devices are being actively forwarded.
An electroluminescent device with the laminate structure has a structure in which an organic luminescent material and a charge transporting organic material (charge transport material) are layered on an electrode and holes and electrons of the respective organic materials move in the charge transport material and recombine to luminesce. As the organic luminescent materials, organic dyes that emit fluorescence such as 8-quinolinol aluminum complexes and cumarin compounds are used. Furthermore, as the charge transport materials, diamino compounds such as N,N-di(m-tolyl)N,N′-diphenyl benzidine and 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane and 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenyl hydrazone compounds can be included.